The present invention relates generally to a spot welding method and apparatus and, more particularly, to such method and apparatus specifically adapted to prevent and/or minimize tip sticking, i.e. sticking of the welding tip to the workpieces, during ultrasonic vibratory spot welding.
Ultrasonic vibratory spot welding processes for joining together two or more similar or dissimilar materials have been used for a number of years. Until recently, however, such methods were limited to use on thermoplastics, non-woven fabrics and metals where weld strength and integrity were not particularly important. This limitation was due, in large measure, to the problems associated with the ultrasonic welding methods employed, most of which were in prototype stages. In those instances when weld strength and weld integrity were important, i.e., when joining together structural aircraft panels and the like, resistance spot welding procedures were used.
Ultrasonic spot welding procedures have recently demonstrated strong potential for improved sheet metal assembly at reduced cost when compared with resistance spot welding and adhesive bonding techniques. Early studies have indicated that welds effected using prototype ultrasonic welding equipment such as, for example, a Sonobond M-8000 ultrasonic spot welder, were superior to welds produced using conventional resistance spot welding procedures. These early trials indicated that for virtually any material combination, an ultrasonically produced spot weld has an ultimate yield strength of more than 2.5 times that of a weld produced using resistance spot welding equipment. Further tests indicated that ultrasonically produced spot welding can be accomplished with a 75% time and cost savings over conventional adhesive bonding techniques. Until now, however, ultrasonic spot welding for large structural metal parts was not possible in a production environment because of the numerous problems associated with the procedures.
Ultrasonic vibratory welding is a metallurgical joining technique which utilizes high frequency vibrations to disrupt the surface films and oxides and which, therefore, promotes interatomic diffusion and plastic flow between the surfaces in contact without any melting of the materials. Briefly stated, the ultrasonic welding process consists of clamping or otherwise securing together the workpieces under moderate pressure between the welding tip and a support anvil and then introducing high frequency vibratory energy into the pieces for a relatively short period of time, i.e., from a fraction of a second to a number of seconds. In many instances, the pieces to be welded are also adhesively bonded together by the insertion of an adhesive bonding agent between the juxtaposed pieces before welding which result in a high strength, uniform joint with superior static and fatigue properties.
One example of an ultrasonic spot welder particularly adapted for use on structural metal workpieces is the Sonobond Model M-8000 Ultrasonic Spot Welder marketed by Sonobond Corporation of West Chester, PA. This welder includes a transistorized, solid state frequency converter which raises standard 60 Hz electrical line frequency to 15-40 kHz and then amplifies the output. The high frequency electrical power travels through a lightweight cable to a transducer in the welding head where it is converted to vibratory power at the same frequency. The vibratory power is, thereupon, transmitted through an acoustic coupling system to the welding tip and then through the tip into and through the workpieces, with the vibratory energy effecting the weld.
The Sonobond M-8000 Ultrasonic Spot Welder includes a wedge-reed, transducer coupling system which transmits lateral vibrations of a perpendicular reed member attached to it so that the welding tip at the upper end of the reed executes shear vibrations on the surface of the workpieces. The transducer includes piezoelectric ceramic elements encased in a tension shell assembly and operates at a nominal frequency of 15 kHz. A solid state frequency converter with a transistorized hybrid junction amplifier powers the welder. The converter operates at a nominal frequency of 15 kHz with a power output variable up to about 4000 RMS RF watts. The welder may be tuned to a precise operating frequency. The frequency converter includes a wide-band RF power measuring circuit which samples output power and detects forward power and load power based on the principle of bi-directional coupling in a transmission line. The signal is processed electronically to provide true RMS values which are selectively displayed on an LED panel meter as either the forward or load power. Forward power is the output of the frequency converter delivered to the transducer in the welding head while load power is the transducer drive power acoustically absorbed in the work zone. The difference between the two readings is the reflected power induced by the load impedance mismatch and is minimized during the welding operation by impedance matching techniques.
In early trials using the prototype ultrasonic welding equipment, a serious "tip sticking" problem was encountered. The welding tip of the welder tended to adhere to the workpiece surfaces. The welding tips and anvils used in ultrasonic welding systems are considerably harder than the workpieces being welded and, as a result of both this and the metal flow which is induced by the vibratory power and clamp force application, the hardened welding tip oftentimes became smeared with the softer welded sheet. In early trials using the prototype ultrasonic welding equipment, the welding zones on the workpieces were characterized by torn and beaten aluminum and aluminum particles being transferred from the workpieces to the surface of the welding tip. After five or six welds, the material transfer tended to accelerate and the surface conditions of both the workpieces and welding tips deteriorated. As a result, it was found that the effective radius of the welding tip was enlarged by the build-up of material with a strong bond occurring between the welding tip and the metal sheet. At times, these bonds were as strong as the bonds formed between the metal workpieces being welded together.
Tip sticking occurs as a result of local scuffing in the region of the contact area where the contact pressure is minimal. It also appears to be associated with a flexural condition where the sheet at the edge of the spot repeatedly rises up and strikes the welding tip producing a flapping action. The problem is, however, less pronounced when welding thicker sheets, thus confirming this theory.
One solution to this "tip sticking" problem is to operate the welder at low power levels. This solution has, however, proved self-defeating since it precludes the generation of strong welds. A second solution is to replace the welding tips every fifth weld and clean them in a sodium hydroxide solution. Obviously, this second solution is not feasible for use in a production environment. The use of welding tips having different configurations and/or fabricated from different materials have also been tried. All of these attempts, however, have proven unsuccessful in overcoming this tip sticking problem.
The present invention utilizes the placement of one or more shims between the welding tip and the workpieces and/or between the anvil and the workpieces. The high frequency energy which is emitted from the welding tip then passes through the shim and into the workpieces causing a weld to occur not only between the workpieces but as well between the shim and the workpiece. The bond which occurs between the welding tip and the shim and/or between the anvil and the shim is a weak bond and is easily broken so as to permit a peeling away of the shim from the workpieces when welding is completed. A strong bond, however, occurs between the workpieces.
Somewhat similar approaches to this problem have been tried in the past. For example, U.S. Pat. No. b 3,533,155, which issued on Oct. 13, 1970 to A. Coucoulas, teaches the bonding of minute electronic leads using a compliant medium. This technique utilized extremely low energy, i.e. 1 watt, to weld very soft material, i.e. gold. Soft aluminum was used as the compliant medium. In contrast, the method of the subject invention is directed to effecting structural bonds between strong structural alloys capable of carrying in excess of a ton of load in shear and uses 4000 watts to effect the weld. The soft compliant materials taught by Coucoulas are substantially different from the hard, non-compliant shims used in the present invention.
Against the foregoing background, it is a primary object other present invention to provide a method for preventing tip sticking during welding operations.
It is another object of the present invention to provide a method of maintaining power tips and anvils free of material pick-up from the workpieces.
It is yet another object of the present invention to provide such a method particularly adapted for use in association with ultrasonic or vibratory welding equipment.
It is still another object of the present invention to provide such a method which may be used in a production environment and which does not deleteriously affect the quality of the resultant weld.
It is yet still another object of the present invention to provide such a method which is relatively inexpensive and which may be used in a production environment.
It is still yet another object of the present invention to provide apparatus for effecting such methods.